Conventional photovoltaic solar cells typically have a conversion efficiency from solar radiation to electricity of not much greater than about 15%. While a large portion of the inefficiency of a solar cell is due to quantum efficiency limitation, i.e., the percentage of incident-radiation that a photovoltaic material can be converted into electricity by creating an electron-hole pair in the photovoltaic material of the solar cell, some of the inefficiency is due to reflection of incident radiation as well as the sub-optimal angle of incidence of radiation reaching the photovoltaic material. Therefore, significant improvements in the efficiency of a solar cell may be possible by decreasing the reflection of incident radiation and by improving the angle of incidence of radiation that approaches the solar cell at a sub-optimal angle.
A process for making a conventional solar cell is shown in FIG. 1. Silicon wafers are cleaned with industrial soaps and then etched using hot sodium hydroxide to remove saw damage. The wafers are further etched in a hot solution of sodium hydroxide and isopropanol to form square-based pyramids; this etching step is sometimes called texturization. Texturization helps reduce the reflection of sunlight. Left untreated, the surface of a photovoltaic cell can act like a mirror, reflecting more than 30% of the light that strikes it.
Since the wafers are pre-doped with boron (p-type), an n-type material is diffused into the wafer, to achieve n-p junction. Phosphorous is the a common diffusant, and diffusion of phosphorous is achieved by subjecting the wafers at high temperature to a phosphorous source. However, deposited phosphosilicate glass may form during the diffusion process, and must be removed.
To further reduce the surface reflection of incident light, an anti-reflection coating (ARC) including a material such as silicon nitride or titanium oxide is applied on the surface. Anti-reflection coatings increase the amount of light coupled into the solar cell. Silicon nitride has gradually replaced titanium dioxide as a preferred anti-reflection coating because of its excellent surface passivation qualities, which prevent carrier recombination at the surface of the solar cell.
Silver is the most widely used metal for contact formation with the silicon-based photovoltaic cell, due to its solderability. Silver in the form of a paste is screen-printed onto the front and the rear of the cell. In addition, aluminum paste is also used onto the rear to achieve a Back Surface Field (BSF), which improves the performance of the solar cell. The paste is then fired at several hundred degrees Celsius to form metal electrodes in ohmic contact with the silicon.
A typical silicon solar cell reflects about one-third of, the incident light that could potentially be converted into electricity.